Friction materials are widely used in brakes to slow or arrest motion of vehicles or moving machinery, and in clutches to impart motion to vehicles or mechanical parts. In every application, other than as a parking brake, the friction material working against a drum, disc, or wheel, usually of cast iron or steel, transfers the energy of motion into heat, resulting in an increase in the temperature of the friction element and of the opposing surface. A most essential and desirable characteristic of any brake or clutch is uniformity of friction over a wide range of conditions, particularly temperature, throughout the life of the material. The next in importance is durability -- long life.
Most friction materials consist of asbestos fibers, mineral fillers, both of which are eminently heat stable. Friction modifiers such as metal particles, abrasives, ground rubber, and ground polymerized and cured cashew nut shell oil, otherwise known as cardolite dust. All of these components are bound together by a resinous or elastomeric binder; it may be a phenol-formaldehyde resin alone or one modified or blended with cross linkable oils such as drying oils, rubber, natural or synthetic, or may be entirely an elastomer of which there is an endless variety. A degree of softness may be imparted to the phenolic resin by substituting meta para cresols or other modified phenols for part of the phenol.
One of the reasons for thus softening the binder is to impart a degree of flexibility which allows the friction material to conform to the opposing surface thereby increasing the friction -- the difference between an artgum eraser and a piece of hard wood sliding over a polished metal plate. A second reason is to improve the wear life of both the friction element and the drum or disc. A brittle material wears more rapidly, one might speculate, because friction results only from particles breaking away from and rolling between the surfaces; with a softer more rubbery bond the surfaces can distort and rebound, thus doing work and absorbing energy. The one problem with more bond or a softer bond is that thermal decomposition by the heat generated under high energy operation at the interface results in a loss of friction commonly known as "fade" when, even with added shoe pressure, the brakes may be unable to control the velocity of the vehicle.
After a friction material has been in use for some time it will be observed that the composition of the surface which rubs against the drum or disc is no longer the same as the substrate. At one extreme of service, such as in very light use, where the temperature never exceeds 200.degree. C. the top 10 to 20 thousandths of an inch will be higher in bond, dark and shiny and partially carbonized; if the top 10 thousandths is machined off it will be higher in acetone extract than material below it. This glazed surface will reduce friction even when the brake is cold.
An intermediate and most satisfactory surface results from moderate conditions -- usually in the range of 200.degree. - 400.degree. C. at the surface with a few peaks over 500.degree. C. and soaking temperatures of 175.degree. - 275.degree. C. The surface will be dark grey to black, high in carbon and low in acetone extractables, in the range of 2 - 3%; the substrate will be little altered from the original composition.
A third and very unsatisfactory condition arises when the friction material has been grossly overheated with surface temperatures over 1000.degree. C. at times and long soaking periods with 400.degree. C. to 650.degree. C. temperatures. The surface will be light grey to white and can be scraped off with a fingernail, showing raw asbestos fiber. Under these conditions friction is savage and wear is drastic. These results are found in brakes that are maladjusted so that they drag continuously and on vehicles that are heavily overloaded. The surface of the friction material is almost devoid of binder or organic filler and the acetone extracts are near zero.
The process that operates within the friction material as it changes from the original homogenous composition to any condition within the range described above can be studied by analysis of successive thin layers turned off, starting with the working surface. From such studies one learns that there is a graded depolymerization of the resinous binder to smaller molecules and at the same time at some levels under certain temperatures, reformation of larger, more complex molecules. Some of the products of these processes are hydrocarbons of a size range that may be oily and good lubricants resulting in the condition known in the industry as "fade", a sudden and dangerous loss of friction.